KR100333067B1 - Method for reducing oscillation marks on stainless steel strip - Google Patents

Abstract

PURPOSE: A method for reducing oscillation marks on stainless steel strip is provided to effectively reduce surface defects of stainless steel products by controlling the number of vibrations of a mold and supply cycle of molten steel from tundish to the mold and stabilizing level of molten steel. CONSTITUTION: In continuously casting stainless steel using an ordinary vibrating mold control device for controlling a mold so that the mold is moved to certain amplitude of vibration and certain number of vibrations by a vertical vibrating means, and an ordinary molten steel level control device for detecting and controlling level of molten steel in the mold using a molten steel level recognition means installed on the side part of the mold and a hydraulic unit for opening or closing nozzle at the lower part of tundish, the method for reducing oscillation marks on stainless steel strip is characterized in that stainless steel is continuously cast in the state that the number of vibrations (f) of the mold is maintained so that a ratio (f/f') of the number of vibrations of the mold (f) to the number of vibrations of molten steel level in the mold (f') is 11 or more, a ratio (A'/A) of the amplitude of vibration of the molten steel level (A') to the amplitude of vibration of the mold (A) is 1.5 or less, and negative strip time (Tn) obtained by the following mathematical expression 1 does not exceed 0.14 second: £Mathematical Expression 1|Tn=1/f{1-1/πCos¬-1(-Vc/(2πAf))} where Tn is negative strip time (sec) per one cycle of the number of vibrations of the mold, f is the number of vibrations of the mold (cycles/min), Vc is casting speed (m/min), and A is the amplitude of vibration of the mold (m).

Description

How to reduce defects in stainless steel cast oscillation marks

The present invention stabilizes molten steel supply control cycle and molten steel level from tundish to mold during continuous casting of stainless steel, and improves defects of cast oscillation mark, resulting in surface defects of stainless steel products. The present invention relates to a method for reducing stainless steel cast oscillation mark defects that can effectively reduce the number of defects.

In general, the steelmaking process of stainless steel is produced by melting molten iron and ferroalloy in an electric furnace, and then securing the desired composition and temperature of the molten steel in a refining furnace, and then the molten steel is transferred to a continuous casting process by ladleing it. It is supplied to the mold cooled by water to solidify to produce slabs (slab, bloom, billet).

1 is a schematic sectional view schematically showing a situation in a mold during continuous casting.

The molten steel 5 is supplied to the mold 2 through the immersion nozzle 1 of the tundish, and the mold 2 is cooled by the coolant so that the molten steel 5 solidifies from the wall of the mold 2 and solidifies. (4) is formed.

On the other hand, the mold powder 3 is added as a lubricant between the mold 2 and the solidification cell 4 during continuous casting, and the molten steel level 6 represents the surface of the molten steel 5 in the mold 2. .

The mold 2 has a constant frequency f and amplitude A to vibrate up and down reciprocating motions to prevent the solidification cell 4 from sticking. As a result, the mold surface is shown in FIG. Likewise, there is a mark of oscillation mark periodically.

When analyzing the cross section of such an oscillation mark part, as shown in FIG. 2B, an oscillation mark defect may exist, and the double segregation defect 9 is 0.5-1.5 mm in depth, and in the case of stainless steel, Ni, P, S and the like have considerably more concentrations than the average composition of the steel, and sometimes there are crack defects 10 in these segregation zone defects 9 and the depth reaches 0.5-1 mm.

The above oscillation mark defects are very small in the case of stainless steel, and the scale off amount of the surface oxidized in the heating furnace is very small. If the depth is 0.3 mm or more, the surface defects of the final product (hot rolled coil or cold rolled coil) ( It is well known to develop into slivers, etc.).

FIG. 3 shows the casting speed and casting speed as a function of time, where the casting speed 12 is the moving speed of the mold 2 which normally vibrates up and down, forming a sine curve, where the speed Since the criterion for the direction of is that the rise time of the mold 2 is set to '+' and the fall time is set to '-', the casting speed 13 naturally shows a constant value as '-'.

The negative strip time (Tn) is the time ('-' direction based on which the rate of falling of the mold (2) is greater than the drawing speed of the cast, that is, the casting speed (13), based on one cycle of vibration of the mold (2). Is defined as

When the negative strip time Tn becomes longer, the compressive stress caused by the vibration of the mold 2 increases in the coagulation cell 4, and the initial coagulation cell 4 is destroyed, so that the solid and liquid coexist in the coagulation cell 4. The molten steel 5 in the region is discharged toward the mold 2 to form segregation zone defects 9 or crack defects in which Ni, P, S, etc., as shown in FIG. Since (10) takes place, it is advantageous to reduce the possible negative strip time Tn.

The most widely used method for this purpose is to increase the mold frequency f.

4 shows the relationship between the mold frequency f and the negative strip time Tn. Since the mold frequency f is usually controlled in a range of 100-200 times / min during continuous casting, the mold frequency f is increased. As a result, the negative strip time Tn is reduced.

However, increasing the mold frequency f increases the frictional force between the mold 2 and the solidification cell 4 and reduces the consumption of the mold powder 3, which causes the solidification cell 4, which is the largest accident during continuous casting, to the mold ( 2) Since there is a high risk of breakout in which the molten steel 5 flows outward from being broken outside, increasing the mold frequency f is not a desirable method and has a very dangerous problem.

The present invention, which is invented to solve the above problems, focuses on the phenomenon in which the control period and amplitude of the molten steel level abnormally increase the negative strip time locally in the same mold frequency, thereby appropriately controlling the mold frequency, and thus the risky mold frequency It is an object of the present invention to provide a stainless steel cast oscillation mark defect reduction method that can reduce cast oscillation mark defects without using an increasing method.

1 is a front sectional view showing a situation in a mold during continuous casting of stainless steel;

Figure 2a is a perspective view showing an oscillation mark formed on a cast steel produced by continuous casting of stainless steel,

2B is a cross-sectional view showing a defect present in the oscillation mark portion;

3 is a graph showing mold speed, casting speed and negative strip time according to casting time during mold vibration;

4 is a graph showing the effect of the mold frequency on the negative strip time during the mold vibration,

5 is a graph showing the effect of the variation of the molten steel level in the mold during the mold vibration on the relative negative strip time,

6 is a graph showing the effect of negative strip time on stainless steel cast oscillation mark defects;

7A is a graph showing the effect on the stainless steel oscillation mark defect rate of the ratio of the mold frequency and the control period of the molten steel level in the mold;

7B is a graph showing the effect of the ratio of the amplitude of the molten steel level in the mold to the amplitude of the mold on the stainless steel oscillation mark defect rate;

8A is a graph showing a change in molten steel level during continuous casting in a conventional method;

8b is a graph showing a change in molten steel level during continuous casting when the method of the present invention is applied;

9 is a graph showing a comparison of the occupancy ratio of the normal oscillation mark when the conventional method and the method of the present invention are applied;

10 is a graph showing a comparison between the incidence of M-type sliver defects on the surface of a hot rolled coil of stainless steel when the conventional method and the method of the present invention are applied.

<Explanation of symbols for main parts of drawing>

1: immersion nozzle 2: mold

3: mold powder 4: solidification cell

5: molten steel 6: molten steel level

7: cast 8: oscillation mark

9: segregation defects 10: crack defects

12: Mold Speed 13: Casting Speed

14: Relative Negative Strip Time

15: molten steel level speed 16: actual mold speed

Tn: negative stripping time

Referring to the technical principle of the present invention having the above object as follows.

In investigating cast oscillation marks, oscillation mark defects do not exist in all oscillation marks, but appear with some periodicity, which indicates that negative strip time is influenced by factors other than the vibration of the mold. It has been found that the inventors have discovered that this another factor is the change over time of the molten steel level.

That is, it can be seen that the molten steel level coincides with the control period of the molten steel inflow from the tundish to the mold (the control uses a stopper or a sliding gate method). Confirmed.

Under these conditions, the negative strip time actually acting on the mold will be affected by this level of molten steel, so if the molten steel level drops, the negative strip time will decrease relatively. will be.

FIG. 5 quantitatively shows such a case. The mold speed 12 is shown without considering the change in the molten steel level when the mold frequency is 157 times / min and the mold amplitude is ± 2 mm. Is constant at -0.9 m / min.

On the other hand, the molten steel level speed 15 shows the change over time of the molten steel level when the frequency of the molten steel level is 30 times / min and the amplitude (the molten steel level fluctuation amount) is ± 3 mm, and the relative mold speed 16 is It shows the relative mold velocity in consideration of the change of the molten steel level, which is the mold velocity 12 minus the molten steel level velocity 15.

In this case, the relative negative strip time 14 in one period is larger than the negative strip time Tn when the molten steel level is not taken into account (when the variation in the molten steel level is 0), and this increase in the negative strip time is It is quite large, and the greater the frequency and amplitude of the molten steel level, the greater.

On the other hand, the structure of the conventional vibration mold control apparatus and molten steel level control apparatus to which this invention is applied is briefly demonstrated.

Vibration mold control apparatus to which the present invention is applied, by using a vertical vibration means for converting the rotary motion into a vertical reciprocating motion consisting of a motor (motor) and a cam (cam) device, etc., the mold is connected to this fixed amplitude and frequency And control to have a sine curve. Here, the mold vibration condition is controlled by using the frequency and the upper and lower vibration width, and since the adjustment of the frequency is generally effective and easy, it is generally used to control the frequency optimally rather than the vibration width control.

The molten steel level control device to which the present invention is applied includes molten steel level recognition means of emitting radiation from a γ-ray provided on the side of the mold and recognizing the molten steel level on a probe provided on another side of the mold, It is configured to maintain a constant mold level at all times by controlling the hydraulic device that opens and closes the nozzle under the tundish to maintain the set level of molten steel. The level can be detected and controlled.

EMBODIMENT OF THE INVENTION Hereinafter, the structure and effect | action of this invention are demonstrated.

The present invention is a conventional vibration mold control device for controlling the mold to move with a constant amplitude and frequency by the up and down vibration means, a molten steel level recognition means installed on the side of the mold and a hydraulic device for opening and closing the nozzle of the tundish bottom In continuous casting of stainless steel by using a conventional molten steel level control device which detects and controls the molten steel level by using a ratio, the ratio of the mold frequency f and the frequency f 'of the molten steel level in the mold (f / f'). Value is 11 or more, and the ratio (A '/ A) of the amplitude (A') of the molten steel level to the mold amplitude (A) is 1.5 or less, and the negative strip time obtained by Equation (1) Continuous casting is carried out while maintaining the mold frequency f so that Tn) does not exceed 0.14 seconds.

Hereinafter, a method for reducing defects in cast iron oscillation marks of stainless steel of the present invention will be described in detail with reference to the accompanying drawings.

In the present invention, continuous casting must be performed under the following conditions in order to prevent casting oscillation mark defects during continuous casting of stainless steel.

First, in the continuous casting of stainless steel, the control period of the mold frequency f and the molten steel level 6 in the mold 2, i.e., the ratio f / f 'of the molten steel level is 11 or more. Continuous casting shall be carried out under the condition that the ratio (A '/ A) of the amplitude (A') of the level (6) to the mold amplitude (A) may be 1.5 or less.

Next, the mold frequency f must be maintained so that the negative strip time Tn obtained by the following equation (1) does not exceed 0.14 seconds under the above condition.

Tn: negative strip time (sec) per cycle of mold vibration

f: mold frequency (cycles / min)

Vc: Casting Speed (m / min)

A: template amplitude (m)

First, the present invention was tested by changing the mold frequency f to investigate the effect of negative strip time Tn on the product defect rate.

6 shows the relationship between the change of negative strip time (Tn) and the cast oscillation mark defect rate for stainless steel, wherein the oscillation mark defect rate is the segregation zone defect or crack in the total number of analyzed oscillation marks. It is defined as a percentage of the number of oscillation marks in which defects are present.

As shown in FIG. 6, the oscillation mark defect rate increases greatly when the negative strip time Tn is 0.15 seconds or more, and decreases when the negative strip time Tn is 0.14 seconds or less, so that the negative strip time Tn is 0.15. If it is more than seconds, it can be seen that the compressive stress caused by the mold vibration acts above the threshold that the initial coagulation cell can withstand, causing the coagulation cell to break, causing segregation or crack defects.

Next, the influence of the molten steel level was examined to confirm the concept of the present invention described above.

Fig. 7A shows the oscillation mark defect rate using the value obtained by dividing the mold frequency f by the control period of the molten steel level, that is, the frequency f 'of the molten steel level as the variable f / f'.

If the f / f 'value is 11 or less, the oscillation mark defect rate increases, and if it is higher than that, i.e., the control period of the molten steel level (the number of molten steel injection from the tundish to the mold per unit time, the number of opening and closing of the stopper or sliding gate per hour) It can be seen that the smaller the decrease, and this result is because the negative strip time (Tn) is longer as the frequency f 'of the molten steel level increases as shown in FIG.

7B shows the oscillation mark defect rate using the value obtained by dividing the molten steel level amplitude A 'by the mold amplitude A, and A' / A as a variable.

When the A '/ A value is more than 1.5, the oscillation mark defect rate increases significantly. As a result, the larger the amplitude of the molten steel level, the larger the amplitude of the molten steel level is, as shown in FIG. 5, and thus the negative strip time (Tn). Because it contributed to a partial increase.

Hereinafter, the present invention will be described in detail through specific examples.

FIG. 8A shows that in casting stainless steel using a conventional method, not only the number of times the control of the molten steel level per unit time is too high, but also the amplitude of the molten steel level is ± 2 mm, in which case f / f 'value is 9, A' / A value is 2.0 it can be seen that out of the scope of the present invention.

On the other hand, FIG. 8B shows that the control frequency of the molten steel level per unit time was greatly reduced to 7 times per minute in the case of applying the method of the present invention, and the vibration width of the molten steel level was also small as ± 1 mm. It can be seen that the value is 22, and the A '/ A value is 0.5, which satisfies all of the ranges of the present invention.

FIG. 9 compares the ratio of the normal oscillation marks without defects of the oscillation marks of the stainless steel cast as a result of applying the method of the present invention, and it can be seen that the number is significantly increased to 72% compared to the conventional 32%.

10 is a comparison of the rejection rate due to the M-type sliver defect, which is one of the defects occurring on the surface of a stainless steel hot rolled coil product as a result of applying the method of the present invention, significantly reduced from 13.2% to 3.5%. It is known that M-type sliver defects are closely related to cracks on the surface of cast steel, and this result is interpreted as a result of effectively reducing cracks and segregation defects on cast surface by reducing oscillation mark defects. .

As described above, by using the method for reducing slab oscillation mark defects of the stainless steel of the present invention, it is possible to efficiently reduce the defects of the slab oscillation mark portion without increasing the mold frequency, which may cause the breakout, so that the surface defects are small. It has the effect of producing a product.

Claims (1)

A mold using a conventional vibration mold control device that controls the mold to move with a constant amplitude and frequency by up and down vibration means, a molten steel level recognition means installed on the side of the mold, and a hydraulic device that opens and closes the nozzle under the tundish. In continuous casting of stainless steel using a conventional molten steel level control device which detects and controls molten steel level, The ratio f / f 'of the mold frequency f and the frequency f' of the molten steel level in the mold is set to 11 or more, and the ratio A 'of the molten steel level (A') to the mold amplitude (A). / A) value of 1.5 or less, stainless steel characterized by continuous casting while maintaining the mold frequency (f) so that the negative strip time (Tn) obtained by the following equation (1) does not exceed 0.14 seconds How to reduce flaw oscillation mark defects in steel. [Equation 1] Tn = 1 / f [1-1 / πCos ^-1 (-Vc / (2πAf))] Tn: negative strip time (sec) per cycle of mold vibration f: mold frequency (cycles / min) Vc: Casting Speed (m / min) A: template amplitude (m)

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